Ion mobility spectrometry is a technique that separates and detects electrically charged particles (e.g., ions) that have been sorted according to how fast they travel through an electrical field in a chamber containing a gas, typically at atmospheric pressure. Small ions travel through the gas faster than do large ions (due to viscous effects) and reach the end of the chamber first, with successively larger ions arriving later. Because ion mobility spectrometry only sorts ions by size (i.e., cross-sectional area per unit charge), and not by their chemical properties or other identifying features, it cannot be used in all cases to make a positive identification of unknown compounds. However, ion mobility spectrometers can be used with certain compounds and can make measurements quite rapidly (e.g., in only a few seconds), therefore making them highly desirable for use in certain applications (e.g., warning and/or screening devices). For example, ion mobility spectrometers are commonly used to detect explosives, narcotics, and chemical warfare (e.g., nerve and blister) agents.
A typical ion mobility spectrometer comprises an ionization region, a drift chamber, and a detector. The ionization region is located at one end of the drift chamber, while the detector is located at the other end of the drift chamber. The ionization region is typically provided with a radioactive source, such as 63Ni, suitable for ionizing the sample material, although other ionizing techniques may be used. Ions of the sample material from the ionization region are introduced into the drift chamber (e.g., either by a fixed open time shutter or by a fixed width gating design), whereupon they ultimately reach the detector at the far end. The arriving ions cause the detector to generate electrical signal peaks proportional to the rate of arriving ions which may thereafter be interpreted to form a conclusion about the nature of the sample material.
While ion mobility spectrometers of the type just described work well and are being used, they are not without their disadvantages. For example, ion diffusion and ion charge-repulsion effects tend to diminish the sensitivity and resolution of an ion mobility spectrometer. Therefore, a need remains for an ion mobility spectrometer and ion mobility spectrometry method that achieve higher resolutions and/or sensitivities when compared with currently available designs.